This invention relates to a channel grouping system and method for a wireless communications system.
In a wireless communications system, an uplink channel between a mobile station and a base station has a received signal strength (RSS) value that depends upon the location of the mobile station relative to the base station. RSS values are associated with electromagnetic transmissions transmitted by mobile stations distributed throughout a coverage area (or xe2x80x9ccell). In a time-division multiple access (TDMA) communications system, multiple uplink channels may be used simultaneously.
The dynamic range of a cell represents a range from a lowest received signal strength of one channel to a highest received signal strength of another channel, where the channels are received simultaneously. For example, a lowest received signal strength can result from a mobile station located at a periphery of the cell served by a base station centrally located in the cell. A wide dispersion of mobile stations throughout a coverage area results in a wide dynamic range.
In a TDMA wireless system, a wide dynamic range creates stringent isolation requirements in base station receiver circuitry. For example, a receiver may group channels from different frequency groups for input into a common intermediate frequency (IF) circuit module. The common IF circuit module requires a degree of isolation that generally increases as the dynamic range of received signal strengths within a cell increases. Radio frequency (RF) shielding can isolate the module circuitry to prevent a xe2x80x9cloudxe2x80x9d channel with a higher signal magnitude from interfering with a xe2x80x9cquietxe2x80x9d channel of lower signal magnitude. However, while RF shielding may provide adequate RF isolation between circuit components, RF shielding decreases maximum achievable circuit board density and increases cost.
In addition to increasing isolation requirements, a wide dynamic range may impede the ability of the base station to adjust the downlink transmit power transmitted to mobile stations. If downlink power control is required, slot-by-slot power control can be utilized. However, slot-by-slot power control is difficult to perform.
One solution to the above problems, described in the related copending U.S. patent application (mentioned above), is to group the channels allocated to a cell into channel groups in which each channel group includes channel members having similar RSS values. The isolation requirement is reduced when the RF channels, being in differing frequency bands and mapped into the same IF channel, have similar received power levels. A receiver can then assign the grouped channels for input into a common IF module with a lower probability of a loud channel interfering with a quiet channel. Downlink power control is also facilitated in that downlink transmit power can be adjusted according to the RSS values in each channel group.
When allocating channels having similar RSS values into channel groups, it is difficult to ensure equal xe2x80x9cblocking rates,xe2x80x9d a measure of quality of service, across all groups of RF channels. A blocking rate represents a probability that one or more mobile stations requesting access to the communications system will be denied access because of insufficient traffic capacity in the wireless communications systemxe2x80x94i.e., the likelihood that access will be xe2x80x9cblockedxe2x80x9d due to hardware limitations in the base station. A typical target blocking rate is 2%.
Base station hardware (e.g., IF modules) is allocated to channel groups in proportion to the number of channels assigned to each group. Therefore, if the number of channels in a particular group is too low to handle the mobile stations accessing that group, accessing occurrences may be blocked. If the number of channels in a group is too high (exceeds demand), another channel group may experience blocked calls because of the poor allocation of resources.
Therefore, a difficulty in creating channel groups having channels with similar RSS values lies in allocating channels to the channel groups so that the target blocking rate is maintained across all channel groups. A uniform blocking rate is desirable because it indicates that all mobile station users in the cell have an equal probability of placing a successful call.
The related copending application solves the problem of channel allocation by generating a cumulative density function of the RSS values from mobile stations in a cell. Channel groups are then created having a uniform number of channels across all groups, with the channel groups being associated with power ranges of differing size. This method, however, may not be suitable for all wireless systems. The present invention presents an alternative solution to the problems associated with the conventional art.
Therefore, the invention, in part, lies in the recognition that a need exists for a method of allocating RF channels having similar power levels to channel groups within a cell, with a uniform target blocking rate maintained across all channel groups in the cell. The invention also addresses the need for a channel grouping system capable of allocating channels having similar RSS values to a channel group, and maintaining a uniform target blocking rate for all channel groups.
According to an aspect of the invention, channels are allocated to channel groups that are associated with a series of power level intervals. The number of channels in a channel group is dependent upon the number of received signal strength (RSS) values falling within the power level interval associated with that group. Therefore, the number of channels in each channel group will typically not be constant for all channel groups.
According to the invention, a group allocator in a wireless communication system allocates channels to channel groups according to RSS data measured at a measurement location (for example, at a base station) within an RF coverage area (e.g., a cell). The RSS data are obtained from signals transmitted by a plurality of mobile stations distributed throughout the cell.
During a sampling period, received signal strengths of signals transmitted by mobile stations accessing the wireless communication system are measured and classified according to RSS. The RSS values are maintained in an RSS database in the group allocator. Using the RSS data, a histogram is generated in which the number of mobile station accessing occurrences at a base station is plotted versus the RSS value at which they access the base station.
The channel groups correspond to a series of RSS power level intervals. The RSS values from the histogram are associated with the power level intervals (and the respective channel groups) into which the RSS values fall. Once the RSS values are associated with their respective channel groups, the population of each channel group (i.e., the number of RSS values falling within a channel group""s power level interval) is determined. The population of each channel group is then divided by the total number of mobile station accessing occurrences, which yields a probability density function (PDF) value for each channel group.
A base station is typically allotted a specified number of channels. According to the present invention, the PDF value for each channel group is multiplied by the number of channels available to a base station. After rounding and adjusting the product values from the multiplication step, the group allocator allocates a number of channels to each channel group according to the rounded and adjusted product value for each channel group. A channel assignment device assigns mobile stations accessing the wireless system to channel groups based upon the RSS of the signals received from the mobile stations.
The method and system of the present invention provides for dynamic, real-time adjustment of the number of channel groups and the number of channels allocated to each group. Because base stations typically measure RSS continuously, RSS data is available to update the RSS database. Therefore, the channel allocator can periodically reconfigure the channel groups to reflect a change in mobile accessing activity within the cell, and according to other factors affecting the quality of service within the cell.
When channels are allocated to channel groups according to the above method and system, isolation requirements in base station circuitry are reduced while maintaining an essentially uniform blocking rate for all mobile stations in the cell. By allocating a number of channels to each group that is proportional to the accessing probability of the channel group, channel groups with higher accessing probabilities are provided greater capacity with which to accommodate a greater number of accessing occurrences. This method increases the capacity of the wireless communication system, and reduces unsuccessful accessing attempts by mobile stations. In addition, downlink power control is facilitated, in that the base station can adjust downlink power according to the RSS values in each channel group.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.